In late years, because of characteristics of being thin, light in weight and having a high picture quality, a liquid crystal display device has become a display device which can compete with CRT. A liquid crystal display with multicolor and high definition or the like has been marketed. Furthermore, with the demand for large screen size and development of high-vision technique, an enlarging projection type video display device with a liquid crystal projector is being spread. As the driving principle of the liquid crystal display device, there have been a TFT type, a MIM type, a STN type, a TN type and the like. Any of these types uses a pair of polarizer, and the emitted display light is linear polarized light. Accordingly, light reaching an observer is linear polarized light.
However, as a means to reduce asthenopia caused by using a liquid crystal display for a long time, a polarizing filter or a polarized eyeglass is used in some cases. An object of the polarizing filter or the polarized eyeglass is to cut off the incoming light from the polarization component having a direction perpendicular to the transmission axis of the polarizer, and to take only the incoming light from the polarization component having a direction parallel to the transmission axis of the polarizer. However, as described above, since light emitted from the liquid crystal screen of a cellular phone or a personal computer is linear polarized light, if an angle of the polarizing filter or the polarizing eyeglass is inclined, the quantity of light is remarkably reduced and the screen becomes invisible in the extreme case, or the left and right sides look different. Such extreme problems were caused.
In order to avoid such problems, a ¼ wavelength plate for a wavelength is used, whereby linear polarized light is considered to be converted to elliptical polarized light, but it is not possible to apply this principle because the wavelength of light emitted from the display is various. Furthermore, apart from the polarizing filter, there is a technique using a cloud membrane phenomenon, in which a special metal line having a width of from 0.1 to 0.15 mm is coated on a substrate in a network structure, whereby a non-coated portion (ultrafine hole of about 0.02 mm×0.02 mm) through which light is transmitted is formed, thereby causing interference and diffraction phenomenon in the wavelength of the incoming light. However, a fine processing technology has been needed for this technique. Similarly, a filter including dispersed fine polymer particles has a little ability to make linear polarized light to nonpolarized light by scattering effect of a particle, but it has been known that there is hardly any effect when a thin film is formed.
Furthermore, whereas there has been proposed that linear polarized light is converted to nonpolarized light by means of a polymer having an amorphous structure (refer to Patent Document 1), practically the amorphous structure has only a low ability to convert linear polarized light to nonpolarized light. We found for the first time that the polymer made into a crystalline aggregate called a spherulite structure has the ability to convert linear polarized light to nonpolarized light effectively. Furthermore, whereas there has also been proposed an optical filter employing a depolarizing plate using a crystal quartz plate (refer to Patent Document 2), it is not practical to arrange the optical filter on a surface of the liquid crystal.
On the other hand, there has been proposed a liquid crystal display device using an alignment film composed of a polymer having a spherulite structure (refer to Patent Document 3). This liquid crystal display device is arranged between a polarizer and a liquid crystal layer in order to widen a viewing angle. Its purpose is different from that of the optical filter of the present invention. Furthermore, when a polymer film having a spherulite structure is used as an optical filter and linear polarized light is attempted to be converted to nonpolarized light, distortion applied in the production of a film remains in the film, and the anisotropy of refractive index is exhibited. So, it is not possible to obtain an optical filter which can uniformly convert linear polarized light to nonpolarized light similar to natural light.    Patent Document 1: Japanese Patent Laid-open No. 2003-185821    Patent Document 2: Japanese Patent Laid-open No. 1998-10522    Patent Document 3: Japanese Patent Laid-open No. 1994-308496